The generic type of ceramic refractory stoppers comprises a rod-shape stopper body, one end of which being designed for fixation to a corresponding lifting mechanism, while the other end of which being defined by a so called stopper head. The rod-shaped stopper body typically has a central longitudinal axis.
It is well known in steel casting to arrange such a stopper rod, which in many cases is a one-piece-stopper rod, in a vertical position, in order to vary the cross-sectional area of an associated outlet opening of a corresponding metallurgical vessel by said lifting action. Insofar any directions disclosed hereinafter, like “top”, “bottom”, “upper and lower ends” always refer to the vertical use position as shown in the Figures of the attached drawing.
Stopper rods of this type have also been used to introduce a treatment gas, such as an inert gas, i.a. argon, into the hot melt (in particular steel melt) to improve the quality of the melt, for example to remove non-metallic inclusions from the melt.
Insofar reference is made to WO 2006/007672. The known stopper rod comprises:                a rod-shaped stopper body defining a central longitudinal stopper axis, including        at least one fitting for connecting the stopper rod to a gas supply line, and        at least one gas channel, running within said stopper body, from an upper end of the stopper body toward an opposite end of the stopper body and extending into a free outer surface area of a stopper head.        
According to WO 2006/007672 it has been observed that the gas may be contaminated during its passage through the gas channel of the stopper rod.
To overcome this drawback WO 2006/007672 discloses a stopper rod, wherein the wall of the gas channel is provided with a layer of a material which does not produce carbon monoxide at the temperature of use.
It was found that the contamination of the treatment gas cannot be avoided reliably by said inner lining. The reasons are not yet fully understood but include:                the gas (for example Argon, Nitrogen) may still be contaminated by small particles of the lining material, for example by abrasion and/or chemical reaction between the gas (for example in case of Nitrogen) and the lining material.        Without said lining the same problems arise with respect to the refractory material of the stopper rod.        Temperature differences within the stopper rod and/or the gas channel respectively may cause                    condensation effects of the treating gas which changes the gas quality arbitrarily and            depositions onto the wall of the gas channel.                        
This is especially true in cases where parts of the stopper rod are immerged in a hot melt and other parts of the stopper rod project above the melt into a much cooler environment.
Further the known technology does not consider harmful constituents of the treatment gas, for example SiO, other volatile sub-oxides, alkali-compounds or the like which may contribute a blockage of the gas-channel(s) in the stopper head.